The present invention relates to instruments and processes for evaluating filtration devices as to leakage, more particularly for the quantitative fit-testing of respirators by measuring concentrations of particles or other suspended elements, inside and outside of a respirator mask.
There are certain occupations, e.g. firefighting, mining, construction, manufacturing and refining, that involve at least occasional exposure to airborne contaminants that can range from mildly irritating to toxic. Respirators are recommended and frequently are required under regulations of the Occupational Safety and Health Administration (OSHA).
Some respirators reply on a tight-fitting face seal to protect the wearer. This invention is directed at testing that seal. The National Institute for Occupational Safety and Health (NIOSH) has classified particulate air-purifying respirators according to 42 CFR Part 84. Three major classes exist within this new standard: class 95, 99 and 100. This invention, however, is directed to air-purifying respirators that rely on the surrounding environment as a source of breathing air. These respirators, designed to remove contaminants from the ambient air, are smaller, easier to maintain and less restrictive in the sense of allowing more freedom of movement. The National Institute for Occupational Safety and Health (NIOSH) has classified particulate air-purifying respirators into four groups: single-use; dusts and mists (DM); dusts, mists and frames (DMF); and high-efficiency particulate air (HEPA) filters.
While the effectiveness of a respirator depends in part on the efficiency of the filter or filters involved, the respirator fit also is of paramount concern. A poorly fitting respirator allows contaminants to flow into the breathing compartment formed by the mask, usually as a wearer inhales. Leakage occurs primarily along the interface of the mask with the face of the wearer, where a properly fitting mask forms a tight seal. A variety of factors can contribute to a poor respirator fit, including selection of a mask of incorrect size or shape, a fault along the edge of the mask intended to form the seal, improper technique in wearing the mask, and facial hair. A poorly fitting respirator mask can lead to considerable exposure to contaminants.
Accordingly, various regulatory agencies have established requirements for the fit-testing of respirators, and standards for determining whether a given respirator fit provides an acceptable seal against leakage.
There are several known approaches to respirator fit-testing. One, known as qualitative fit-testing, relies on the subjective response of an individual wearing the respirator upon exposure to an odor-producing aerosol, such as smoke or a suspension of liquid droplets, e.g. banana oil. Quantitative fit-testing is considered more accurate and more reliable. A common method of quantitative fit-testing involves taking particle concentration measurements, both inside of a respirator mask and just outside of the mask. This is accomplished by using a vacuum pump to draw an aerosol sample from the atmosphere just outside the mask, and then drawing another aerosol sample from within the mask, either by using a sampling adapter or a test mask with a face piece modified to receive a sampling tube. The two samples are provided, alternatively, to a condensation particle counter (also known as a condensation nucleus counter) that generates particle counts indicating the respective concentrations of the tested aerosol samples.
The two counts are compared by providing a ratio of the count outside the mask to the count inside the mask, known as the "fit factor". A higher fit factor indicates a filtration that effectively seals against leakage.
The validity of this test is based largely on an assumption that the count or concentration inside the mask is due to leakage rather than penetration through the filter, i.e. an assumption that the filter is nearly 100 percent efficient.
The accuracy of this assumption depends upon the type of filter involved, and the size of the particles or other suspended elements. Both high efficiency filters and low efficiency filters have efficiencies that vary with particle size. More particularly, each filter has a minimum efficiency (corresponding to a maximum particle penetration rate) at a midpoint along a particle size spectrum. Efficiency rises (reflecting reduced penetration) in both directions from the midpoint. Typically the midpoint occurs within a particle size range of 0.1 to 0.3 microns.
HEPA filters are at least 99.97 percent efficient, even at the minimum-efficiency midpoint. Other filters are considerably less efficient. For example, FIG. 1 shows on a log/log scale a fractional filtration efficiency curve representative of lower efficiency filters. The curve shows a minimum efficiency slightly over 92 percent at a particle size slightly less than 0.2 microns. For particles exceeding 0.5 microns or less than about 0.045 microns, the efficiency exceeds 99.9 percent.
While actual curves and values will vary depending on the filter class and the brand of filter within a given class, it is clear that when a class 95 respirator is exposed to a polydisperse aerosol over the size spectrum illustrated in FIG. 1, a relatively large number of particles at and near the most penetrating size pass through the filter and are detected inside the respirator mask. In practice, the number of particles entering the mask through the filter is substantially larger than the number entering the mask due to face-seal leakage. The result is a severe distortion of the calculated fit factor, erroneously indicating a poor fit when the respirator in fact may fit properly. As a result, class 99 or class 100 filters are either recommended or required for respirator fit-testing, even when the respirators involved are intended for use with class 95 or other less efficient filters.
Those of skill in the art are aware of this problem, and increasingly concerned because lower efficiency filters have gained acceptance for a wider range of uses. A related concern is exemplified by a class of respirators known as "N95" filtering facepieces. In their most common configuration, these respirators consist of a mask composed entirely of the filter medium, without a supporting elastomeric mask. The N95 respirator is greater than 95 percent efficient at the most penetrating particle size. These respirators have penetration sufficient to overwhelm the particles coming through leaks, leading to inaccurate results if the fit-testing is conducted in a polydisperse aerosol environment. Recent regulatory changes have resulted in an upsurge of N95 respirator production and usage, and government regulations continue to require fit-testing.
In view of these difficulties, researchers in this area have tried several approaches to fittesting respirators without HEPA filters. One approach involves generating a suitable monodisperse aerosol, e.g. with all particles at or about 2.5 micrometers in diameter. Dust/mist filters, N95 filters, and other low efficiency filters are considerably more efficient with respect to particles at or near 2.5 microns in diameter. Results based on this type of testing, however, are reliable only if testing occurs within a controlled atmosphere including only the monodisperse aerosol. Maintaining this atmosphere is expensive, requiring an aerosol generator to produce the monodisperse aerosol and a chamber or other enclosure surrounding the person wearing the respirator under test. The enclosure limits the individual's ability to perform certain exercises or movements during fit-testing. This technique is described in an article entitled "Validation of a Quantitative Fit-Test for Dust/Fume/Mist Respirators: Part I", Iverson et al; Applied Occupational Environmental Hygiene, March 1992, pp. 161-167.
Another approach is based on the discovery that for DM and DFM respirators, the relationship between filter penetration and leakage depends upon the face velocity (flow rate). The approach is described in an article entitled "Fit-Testing for Filtering Face Pieces: Search for a Low-Cost, Quantitative Method", Myojo et al; American Industrial Hygiene Association Journal, 55 (9), 1994, pp. 797-805. Tests were conducted on mannequins and human subjects, both breath-holding and normal breathing. The technique, however, is limited primarily to aerosols in the submicrometer size range. Also, the reliability of tests on human subjects breathing normally depends on the ability to predict and monitor the subject's inhalation rate.
Another known fit-test involves using an optical particle counter in combination with lower efficiency filters, such as dust/mist and N95. The complete polydisperse aerosol is sampled. Due to the limited capacity of the optical particle counter, i.e. its ability to detect only relatively large particles (more than 0.5 microns in diameter), the tendency of penetrating particles to bias leakage test results is reduced. However, the relatively small number of large particles occurring naturally in ambient conditions limits the utility of this approach, because the number of sensed particles is not sufficient to afford statistical accuracy.
Therefore, it is an object of the present invention to provide a process for testing filtration devices for leakage, in which reliable results can be obtained based on aerosol sampling in ambient, naturally occurring conditions.
Another object is to provide a system for leak-testing filtration devices, that does not requires either a device for generating a prescribed artificial atmosphere or an enclosure for keeping an individual within an artificial atmosphere during testing, although it may advantageously employ a polydisperse aerosol generator in certain cases.
A further object is to provide a respirator fit-testing system that allows more freedom of movement for the individual during testing, to facilitate duplication of on-the-job tasks and movements.
Yet another object is to provide a respirator fit-testing process based on sampling polydisperse aerosols under ambient conditions, then selecting a predetermined range of particle sizes within the sampled aerosols to improve statistical accuracy and avoid biasing of leak-test results due to particle penetration.